Allow implementing custom brush types

[JimBobSquarePants]

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Description

Fixes #20 allowing custom brushes inheriting BrushApplicator

[codecov]

Codecov Report

Merging #149 (f4fdab3) into master (423bf06) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #149   +/-   ##
=======================================
  Coverage   69.35%   69.35%           
=======================================
  Files          89       89           
  Lines        5142     5142           
  Branches     1051     1051           
=======================================
  Hits         3566     3566           
  Misses       1359     1359           
  Partials      217      217           

Flag	Coverage Δ
unittests	69.35% <100.00%> (ø)

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Impacted Files	Coverage Δ
...c/ImageSharp.Drawing/Processing/BrushApplicator.cs	100.00% <100.00%> (ø)

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[antonfirsov]

LGTM